package org.apache.lucene.codecs.perfield;

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.ServiceLoader; // javadocs
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;

import org.apache.lucene.codecs.FieldsConsumer;
import org.apache.lucene.codecs.FieldsProducer;
import org.apache.lucene.codecs.PostingsFormat;
import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.index.Fields;
import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.SegmentWriteState;
import org.apache.lucene.index.Terms;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.RamUsageEstimator;

import static org.apache.lucene.index.FilterAtomicReader.FilterFields;

/**
 * Enables per field postings support.
 * <p>
 * Note, when extending this class, the name ({@link #getName}) is 
 * written into the index. In order for the field to be read, the
 * name must resolve to your implementation via {@link #forName(String)}.
 * This method uses Java's 
 * {@link ServiceLoader Service Provider Interface} to resolve format names.
 * <p>
 * Files written by each posting format have an additional suffix containing the 
 * format name. For example, in a per-field configuration instead of <tt>_1.prx</tt> 
 * filenames would look like <tt>_1_Lucene40_0.prx</tt>.
 * @see ServiceLoader
 * @lucene.experimental
 */

public abstract class PerFieldPostingsFormat extends PostingsFormat {
  /** Name of this {@link PostingsFormat}. */
  public static final String PER_FIELD_NAME = "PerField40";

  /** {@link FieldInfo} attribute name used to store the
   *  format name for each field. */
  public static final String PER_FIELD_FORMAT_KEY = PerFieldPostingsFormat.class.getSimpleName() + ".format";

  /** {@link FieldInfo} attribute name used to store the
   *  segment suffix name for each field. */
  public static final String PER_FIELD_SUFFIX_KEY = PerFieldPostingsFormat.class.getSimpleName() + ".suffix";

  /** Sole constructor. */
  public PerFieldPostingsFormat() {
    super(PER_FIELD_NAME);
  }

  /** Group of fields written by one PostingsFormat */
  static class FieldsGroup {
    final Set<String> fields = new TreeSet<String>();
    int suffix;

    /** Custom SegmentWriteState for this group of fields,
     *  with the segmentSuffix uniqueified for this
     *  PostingsFormat */
    SegmentWriteState state;
  };

  static String getSuffix(String formatName, String suffix) {
    return formatName + "_" + suffix;
  }

  static String getFullSegmentSuffix(String fieldName, String outerSegmentSuffix, String segmentSuffix) {
    if (outerSegmentSuffix.length() == 0) {
      return segmentSuffix;
    } else {
      // TODO: support embedding; I think it should work but
      // we need a test confirm to confirm
      // return outerSegmentSuffix + "_" + segmentSuffix;
      throw new IllegalStateException("cannot embed PerFieldPostingsFormat inside itself (field \"" + fieldName + "\" returned PerFieldPostingsFormat)");
    }
  }
  
  private class FieldsWriter extends FieldsConsumer {
    final SegmentWriteState writeState;

    public FieldsWriter(SegmentWriteState writeState) {
      this.writeState = writeState;
    }

    @Override
    public void write(Fields fields) throws IOException {

      // Maps a PostingsFormat instance to the suffix it
      // should use
      Map<PostingsFormat,FieldsGroup> formatToGroups = new HashMap<PostingsFormat,FieldsGroup>();

      // Holds last suffix of each PostingFormat name
      Map<String,Integer> suffixes = new HashMap<String,Integer>();

      // First pass: assign field -> PostingsFormat
      for(String field : fields) {
        FieldInfo fieldInfo = writeState.fieldInfos.fieldInfo(field);

        final PostingsFormat format = getPostingsFormatForField(field);
  
        if (format == null) {
          throw new IllegalStateException("invalid null PostingsFormat for field=\"" + field + "\"");
        }
        String formatName = format.getName();
      
        FieldsGroup group = formatToGroups.get(format);
        if (group == null) {
          // First time we are seeing this format; create a
          // new instance

          // bump the suffix
          Integer suffix = suffixes.get(formatName);
          if (suffix == null) {
            suffix = 0;
          } else {
            suffix = suffix + 1;
          }
          suffixes.put(formatName, suffix);

          String segmentSuffix = getFullSegmentSuffix(field,
                                                      writeState.segmentSuffix,
                                                      getSuffix(formatName, Integer.toString(suffix)));
          group = new FieldsGroup();
          group.state = new SegmentWriteState(writeState, segmentSuffix);
          group.suffix = suffix;
          formatToGroups.put(format, group);
        } else {
          // we've already seen this format, so just grab its suffix
          assert suffixes.containsKey(formatName);
        }

        group.fields.add(field);

        String previousValue = fieldInfo.putAttribute(PER_FIELD_FORMAT_KEY, formatName);
        assert previousValue == null;

        previousValue = fieldInfo.putAttribute(PER_FIELD_SUFFIX_KEY, Integer.toString(group.suffix));
        assert previousValue == null;
      }

      // Second pass: write postings
      for(Map.Entry<PostingsFormat,FieldsGroup> ent : formatToGroups.entrySet()) {
        PostingsFormat format = ent.getKey();
        final FieldsGroup group = ent.getValue();

        // Exposes only the fields from this group:
        Fields maskedFields = new FilterFields(fields) {
            @Override
            public Iterator<String> iterator() {
              return group.fields.iterator();
            }
          };

        format.fieldsConsumer(group.state).write(maskedFields);
      }
    }
  }

  private class FieldsReader extends FieldsProducer {

    private final Map<String,FieldsProducer> fields = new TreeMap<String,FieldsProducer>();
    private final Map<String,FieldsProducer> formats = new HashMap<String,FieldsProducer>();

    public FieldsReader(final SegmentReadState readState) throws IOException {

      // Read _X.per and init each format:
      boolean success = false;
      try {
        // Read field name -> format name
        for (FieldInfo fi : readState.fieldInfos) {
          if (fi.isIndexed()) {
            final String fieldName = fi.name;
            final String formatName = fi.getAttribute(PER_FIELD_FORMAT_KEY);
            if (formatName != null) {
              // null formatName means the field is in fieldInfos, but has no postings!
              final String suffix = fi.getAttribute(PER_FIELD_SUFFIX_KEY);
              assert suffix != null;
              PostingsFormat format = PostingsFormat.forName(formatName);
              String segmentSuffix = getSuffix(formatName, suffix);
              if (!formats.containsKey(segmentSuffix)) {
                formats.put(segmentSuffix, format.fieldsProducer(new SegmentReadState(readState, segmentSuffix)));
              }
              fields.put(fieldName, formats.get(segmentSuffix));
            }
          }
        }
        success = true;
      } finally {
        if (!success) {
          IOUtils.closeWhileHandlingException(formats.values());
        }
      }
    }

    @Override
    public Iterator<String> iterator() {
      return Collections.unmodifiableSet(fields.keySet()).iterator();
    }

    @Override
    public Terms terms(String field) throws IOException {
      FieldsProducer fieldsProducer = fields.get(field);
      return fieldsProducer == null ? null : fieldsProducer.terms(field);
    }
    
    @Override
    public int size() {
      return fields.size();
    }

    @Override
    public void close() throws IOException {
      IOUtils.close(formats.values());
    }

    @Override
    public long ramBytesUsed() {
      long sizeInBytes = 0;
      for(Map.Entry<String,FieldsProducer> entry: formats.entrySet()) {
        sizeInBytes += entry.getKey().length() * RamUsageEstimator.NUM_BYTES_CHAR;
        sizeInBytes += entry.getValue().ramBytesUsed();
      }
      return sizeInBytes;
    }
  }

  @Override
  public final FieldsConsumer fieldsConsumer(SegmentWriteState state)
      throws IOException {
    return new FieldsWriter(state);
  }

  @Override
  public final FieldsProducer fieldsProducer(SegmentReadState state)
      throws IOException {
    return new FieldsReader(state);
  }

  /** 
   * Returns the postings format that should be used for writing 
   * new segments of <code>field</code>.
   * <p>
   * The field to format mapping is written to the index, so
   * this method is only invoked when writing, not when reading. */
  public abstract PostingsFormat getPostingsFormatForField(String field);
}
